Tar sands include a combination of clay, sand, water, and bitumen, which is a black viscous mixture of hydrocarbons obtained naturally or as a residue from petroleum distillation. Tar sands can be mined and processed to extract the oil-rich bitumen, and the bitumen can be refined into oil. The recovery of oil from the bitumen in tar sands requires extraction and separation systems to separate the bitumen from the clay, sand, and water that make up the tar sands. Bitumen also requires upgrading before it can be refined. Because it is so viscous, bitumen also requires dilution with lighter hydrocarbons so that it can be transported by pipelines or tank cars.